1. Field of the Invention
The present invention relates to a method for producing a Mn—Zn ferrite having a high saturation magnetic flux density at high temperatures (in the vicinity of 100° C.), a high permeability and a low loss.
2. Description of the Related Art
Downsizing and high powering of electronic devices have been promoted. Accordingly, high density integration and high speed processing of various components have progressed, and thus power supply lines are demanded to supply large electric current.
Additionally, even under high temperatures, demanded are power supply lines which can maintain the predetermined performances. This is because power supply lines are exposed to heat emitted from components (for example, CPU) as the case may be. Additionally, power supply lines are required to maintain predetermined performances under such conditions that the environmental temperature is high as in automobile electronic circuits.
Accordingly, transformers and reactors to be used in power supply lines are also required to be capable of being used with large current even under high temperatures.
As the materials to be used for these transformers and reactors, soft magnetic metal materials and ferrite materials can be cited. Additionally, ferrite materials are classified into Mn—Zn based ferrites and Ni based ferries.
Soft magnetic metal materials are higher in saturation magnetic flux density than ferrites, and hence cause no magnetic saturation even for larger currents flowing therethrough. However, there are problems in that soft magnetic metal materials are generally high in loss, high in price, high in specific gravity, and poor in rustproof property.
On the other hand, ferrites are excellent in cost performance, and have advantage such that loss is low in a frequency range between a few 10 kHz and a few 100 kHz. Additionally, Mn—Zn based ferrites are higher in saturation magnetic flux density than Ni based ferrites. Therefore, for transformers and choke coils used for large current, Mn—Zn based ferrites are generally used.
For example, Japanese Patent Publication No. S63-59241, corresponding to JP 56-005331, has achieved a low loss at 150.degree. C. or higher by including at least one of NiO, Li. sub.2O and MgO in addition to MnO and ZnO. However, this ferrite is not suitable because the temperature at which the loss exhibits the minimum value (hereinafter referred to as bottom temperature) is 150.degree. C. or higher for this material, and accordingly this material causes degradation in loss and initial permeability in a temperature range (between 80 and 120.degree. C.) in which common transformers and cores for use in common reactors are used.
Additionally, Japanese Patent No. 3389170 discloses that substituting a part of a Mn—Zn ferrite with NiO makes the Mn—Zn ferrite excellent in DC pre-magnetization characteristics at a saturation magnetic flux density Bs of 440 mT or more and usable in a wide temperature range.
Moreover, in Japanese Patent Laid-Open No. 2001-080952, by adding CoO to a Mn—Zn ferrite, there have been achieved a drastic decrease in power loss in a wide temperature range of 20 to 100° C. and an alleviation of the temperature variation of the power loss; more specifically, there have been obtained properties such that the minimum power loss is 400 kW/m3 or less and the difference between the maximum and the minimum of the power loss is 150 kW/m3 or less.
Yet additionally, Japanese Patent Laid-Open 11-3813 has shown that decrease of loss can be achieved even in a high frequency range of approximately 1 MHz or more by including NiO and CoO as basic constituents in a Mn—Zn ferrite having a content of Fe2O3 exceeding 50 mol %. This Mn—Zn ferrite has a composition in which 52 to 68 mol % of Fe2O3, 0.5 to 10 mol % of NiO, 15 mol % or less of ZnO, 0.005 to 0.5 mol % of CoO, and the balance substantially being MnO.
Additionally, a ferrite disclosed in Japanese Patent Laid-Open No. 2000-286119 is known as a Mn—Zn ferrite simultaneously containing NiO and CoO. This ferrite has a basic composition including 52 to 56 mol % of Fe2O3, 6 to 14 mol % of ZnO, 4 mol % or less of NiO, 0.01 to 0.6 mol % of CoO, and the balance substantially being MnO, wherein this ferrite further comprises 0.0050 to 0.0500 wt % of SiO2 and 0.0200 to 0.2000 wt % of CaO in relation to the basic composition, and at least one additive, in a predetermined content, selected from the group consisting of Ta2O5, ZrO2, Nb2O5, V2O5, K2O, TiO2, SnO2 and HfO2, and the temperature at which the loss of the ferrite becomes minimum is 50° C. or higher and 85° C. or lower under the measurement conditions of 100 kHz in frequency and 200 mT in maximum magnetic flux density.
As described above, a Mn—Zn ferrite including both of NiO and CoO can be provided with an excellent property of loss. In this connection, although the Mn—Zn ferrite disclosed in Japanese Patent Laid-Open No. 11-3813 is excellent for the loss in a high frequency range of approximately 1 MHz or more, this ferrite is not suitable for use in a frequency range between a few 10 kHz and a few 100 kHz.
Additionally, when even if the saturation magnetic flux density at room temperature is high, the saturation magnetic flux density is decreased in the temperature range (between 80 and 120° C.) in which common transformers and cores for use in reactors are used, the desired performances of the transformers and the cores for use in reactors cannot be displayed.
In the Mn—Zn ferrite disclosed in Japanese Patent Laid-Open 2000-286119, the loss in the frequency range between a few 10 kHz and a few 100 kHz is excellent, but any significant consideration has not been made on the saturation magnetic flux density in the vicinity of 100° C.